1. Field of the Invention
The present invention relates to a retaining device for a one-piece battery. Such batteries are used to power electric vehicles. The capacity of a nickel-cadmium or nickel-hydridable metal one-piece battery is in the range 5 Ah to 250 Ah.
2. Description of the Related Art
In a one-piece battery, the plate groups are contained in compartments in a single casing. The casing is usually made of a plastics material and prismatic in shape. Each plate group is constituted by an assembly of alternate positive and negative planar electrodes separated from each other by a separator and impregnated with an electrolyte. The plate group has two small faces corresponding to the edges of the stacked electrodes and two large faces parallel to the electrode surface. The longitudinal walls of the battery correspond to the small faces of the group and the transverse walls correspond to the large faces of the group.
During cycling, the electrodes undergo dimensional variations which exert a thrust on the casing walls opposite the large faces of the group. Further, the internal gas pressure bears on all of the casing walls. In order to prevent the battery from expanding and deforming, a device must be provided which will retain the walls.
U.S. Pat. No. 4,020,244 describes a battery composed of a plurality of sealed parallelepipedal rechargeable cells, in particular nickel-cadmium type cells, provided with a device intended to prevent the battery from deforming. The fixing system comprises a pair of compression plates at the ends of the stack of cells and a plurality of bands passing around the stack and the plates. The bands are cinched tight and their ends are welded together.
The inevitable release of heat caused by welding necessitates the presence of heatsinks. When the welding is carried out at the ends of the battery, the mass of the plates may be sufficient to fulfil this role; however, if the welding is carried out on one of the sides, heatsinks must be added to protect the rechargeable cells.
Patent FR-1 267 153 describes a battery made up of a plurality of rechargeable cells. The groups are contained in boxes which are juxtaposed and connected by belts associated with the plates. The device includes plate groups perpendicular to their plane. The belts are closed up on themselves by tensioning devices to make it possible to release the belt if it is necessary to remove or introduce a box.
For a one-piece battery, the problem of replacing a box does not arise. A tensioning device would unnecessarily increase the bulk and cost of the battery.